1. Field of the Invention
The present invention relates to a propylene-based polymer, a production method therefor, a composition using the same, and an application thereof, and in more detail, the present invention relates to a propylene-based polymer which is suitably applicable to foam molding, sheet molding, blow molding or the like, because of having a high fluidity at melting, as well as high melt tension, high swell ratio and thus good molding workability, and a production method therefor, a propylene-based polymer composition using the same, and an application thereof.
2. Description of the Prior Art
Conventionally, polypropylene has been used in a wide range in various fields, because of having the characteristics of high melting point, high tensile strength, high rigidity and chemical resistance.
However, usual polypropylene has defect of limitation in usage to foam molding, sheet molding, blow molding or the like, due to low melt tension and melt viscoelasticity.
In order to solve these defects, as a method for the addition of a component for enhancing a melt tension, there have been known a method for mixing of high density polyethylene having high melt tension, which is produced by a chromium catalyst; a method for mixing of low density polyethylene, which is produced by a high pressure radical polymerization method; a method for polymerization of high molecular weight polyethylene before the polymerization stage of propylene; or the like, however, there is a defect of impairing of original characteristics of polypropylene, such as insufficient elastic modulus, strength and heat resistance, or decrease of fluidity, in a component for enhancing a melt tension.
Under these circumstances, there has been an idea of a method for enhancing a melt tension, by cross-linking of polypropylene itself or introduction of a long-chain-branch, and various attempts has been tried. For example, as a method for cross-linking, there are included a method for irradiation of electron beams after polymerization (for example, refer to Patent Literature 1); a method for using a peroxide, or a peroxide and a cross-linking co-agent (for example, refer to Patent Literature 2); and also as a method for introducing a long-chain-branch, a method for grafting a radically polymerizable monomer to polypropylene (for example, refer to Non-Patent Literature 1); a method for copolymerizing a propylene and a polyene (for example, refer to Patent Literature 3, 4); and the like.
However, the method for cross-linking after polymerization has a difficulty in control of a side reaction, which leads to high-order cross-linking, and it has a problem that poor appearance or adverse effect on mechanical characteristics are raised, due to generation of gel, as well as limitation in arbitrary control of molding workability, and control range is narrow. In addition, use of a component having low crystallinity, to enhance branch generation efficiency, raises a problem of impairing of mechanical property or cleanness as a product. On the other hand, the method for grafting of a radically polymerizable monomer impairs chemical stability of polypropylene, and raises a problem of recycling property as well. Further, the copolymerization method with a polyene does not necessarily provide sufficient improvement effect of melt tension, as well as provides difficulty in control of property due to risk of gel generation. In addition, a separation and recovery step of the polyene after completion of the copolymerization is essential, which remains a problem in view of production cost.
In recent years, there has been proposed a macromer copolymerization method utilized mainly a metallocene catalyst. The metallocene catalyst, in the broad sense, is a transition metal compound having at least one conjugated five-member ring ligand, and a ligand having a cross-linked structure is generally used for polymerization of propylene.
There have been disclosed, initially, ethylenebis(indenyl)zirconium dichloride or ethylenebis(4,5,6,7-tetrahydroindenyl)zirconium dichloride (JP-A-61-130314), which were found as a complex that enables to produce an isotactic polyolefin, dimethylsilylenebis substituted cyclopentadienyl zirconium dichloride having a silylene group as a cross-linking group (JP-A-1-301704), dimethylsilylenebis(indenyl)zirconium dichloride (JP-A-1-275609), dimethylsilylenebis(2-methylindenyl)zirconium dichloride having improved stereoregularity and molecular weight to a certain degree by introduction of a substitution group adjacent (at the second site) to the cross-linking group of a cyclopentadienyl compound (JP-A-4-268307), further, dimethylsilylenebis(2-methyl-4-phenylindenyl)zirconium dichloride (JP-A-6-100579) or dimethylsilylenebis(2-methyl-4-phenyl-4-hydroazulenyl)zirconium dichloride (JP-A-10-226712) having still more improved activity, stereoregularity and molecular weight by introduction of an aryl group at the fourth site, and further recently, dichloro(1,1′-dimethylsilylenebis(2-ethyl-4-(3-chloro-4-t-butylphenyl)-4H-azulenyl))hafnium introduced with a specific substitution group at a specific site of the aryl group at the fourth site (JP-A-2003-292518), and a highly bulky hetero substitution group is introduced at the second site (JP-A-2002-194016, JP-A-2002-535339, JP-A-2004-2259, and JP-A-2004-352707); and the like
They are aiming at mainly improvement of catalytic activity, or melting point and molecular weight of the resultant polypropylene, and do not suggest production suitability of a macromer or polypropylene having a long-chain-branch.
As a macromer copolymerization method utilized a metallocene catalyst, there has been contrived, for example, a method (a macromer copolymerization method), by producing a propylene macromer having a vinyl structure at the terminal, at the first stage of polymerization (a synthesis step of a macromer), by using a specific complex under specific polymerization condition, and then by carrying out copolymerization with propylene, by using a specific catalyst under specific polymerization condition at the second stage of polymerization (a macromer copolymerization step), which provides excellent recycling property and has no risk of gel generation for improvement of melt tension, without having high-order cross-linking, nor impairment of original chemical stability as polypropylene (for example, refer to Patent Literature 5, 6).
However, this method requires polymerization by using a specific complex at relatively high temperature and under low pressure, for efficiently obtaining a terminal vinyl structure necessary as the macromer, at the former stage. Therefore, the resultant macromer inevitably becomes a macromer having low molecular weight and stereoregularity. In addition, at the latter stage, copolymerization of the macromer obtained at the former stage with propylene is carried out, however, due to small quantity of the macromer to be copolymerized, relative to charged quantity of the macromer, a macromer having low molecular weight and stereoregularity remains in not-neglectable quantity, in a macromer copolymer, which becomes a product. In addition, there is contained without being copolymerized, a component, which is by-produced at the macromer synthesis step and has a terminal with similarly low molecular weight and low stereoregularity, for example, a saturated terminal other than a vinyl group, resulting in decrease in mechanical property, such as rigidity or impact strength of a product, generation of a problem of stickiness, or making control difficulty of fluidity and molding property. Further, in the case where a macromer having low molecular weight and low stereoregularity remains in a large quantity in a product, for example, when it is used as a container, elution components increase, which inevitably generates a defect of, what is called, poor cleanness.
Against the above multi-stage polymerization method, there has been contrived a single-stage polymerization method, where the synthesis step of a macromer and the copolymerization step of the macromer are carried out at the same time with a specific complex (a macromer generation copolymerization method in situ) (for example, refer to Patent Literature 7).
However, this method does not necessarily provide sufficient generation quantity of the macromer and copolymerization quantity of the macromer, and thus improvement effect of melt property is at an insufficient level. In addition, there is also a problem of poor fluidity, due to narrow molecular weight distribution. In addition, this method requires slurry polymerization of propylene in low concentration for efficient generation of the macromer, and thus not preferable in view of production efficiency and environmental load.
In addition, there has been contrived a method for obtaining a propylene-based polymer having wide molecular weight distribution and high branch quantity, by carrying out a bulk polymerization in a single stage with a catalyst composed of a specific complex and a specific chemically-treated clay (refer to Patent Literature 9).
However, this method does not introduce a controlled branch due to implementation of macromer generation and copolymerization with a single complex only, although it provides a large quantity of branches, and thus improvement effect of melt property is not sufficient.
On the other hand, a production method for a propylene polymer having wide molecular weight distribution or stereoregularity distribution, by using two kinds of complexes, has also be known, for example, there has been reported that polypropylene having a molecular weight distribution of 4.8 to 6.3 is obtained, by using a catalyst composed of a complex, where ethylenebis(indenylhafnium)dichloride and a small quantity of ethylenebis(indenylzirconium)dichloride are mixed, and methylaluminoxane (refer to Patent Literature 8), however, this method provides only two-peak type wide molecular weight distribution and does not provide one introduced with a branch structure by macromer generation and copolymerization. Therefore, improvement effect of melt property is not so high.
In recent years, there has been reported that a propylene-based polymer obtained by multi-stage polymerization shows relatively high melt tension, which is obtained by using a catalyst in combination of two kinds of metallocene complexes, specifically, complexes such as rac-SiMe2[2-Me-4-Ph-lnd]2ZrCl2 and rac-SiMe2[2-Me-4-Ph-lnd]2HfCl2, along with silica which supports methylaluminoxane (MAO) (refer to Patent Literature 10).
However, any of these methods has not yet been commercialized, and thus it has been desired to develop a method for producing a propylene-based polymer, which enables to improve melt property, in an industrially more simple and easy way.    [Patent Literature 1] U.S. Pat. No. 5,541,236    [Patent Literature 2] WO99/27007    [Patent Literature 3] JP-A-5-194778    [Patent Literature 4] JP No. 3260171    [Patent Literature 5] JP-A-2001-525460    [Patent Literature 6] JP-A-10-338717    [Patent Literature 7] JP-A-2002-523575    [Patent Literature 8] JP-A-2-255812    [Patent Literature 9] JP-A-2007-154121    [Patent Literature 10] JP-A-2001-64314    [Non-Patent Literature 1] T. C. Chung et. al., Synthesis of Polypropylene-graft-poly(methylmethacrylate) Copolymers by Borane Approach, Macromolecules, (1993), volume 26, No. 14, page 3467-3471